If Google can certainly do this, why can't Apple?
Google does it because there's too much fragmentation to the point that developers can't test on every device. That isn't the case with Apple.
The problem with demos/trials is that it gives you too much time to hesitate. I have about 20 apps and games on my Windows Phone that I haven't bought yet that are on trial mode. Most people will load up on 100 game demos, and then never buy most of them. So you're wasting them bandwidth AND a sale.